Method and apparatus providing an optical guide in image sensor devices

ABSTRACT

A device and method for providing an optical guide of a pixel to guide incoming light to/from a photo-conversion device of the pixel to improve the optical crosstalk immunity. The optical guide includes an optically reflecting barrier formed as a trench filled with a material which produces reflection. The trench fill material may have an index of refraction that is less than the index of refraction of the material used for the trench surrounding layers to provide a light reflective structure or the trench fill material may provide a reflection surface.

FIELD OF THE INVENTION

The invention relates generally to solid state imaging devices and moreparticularly to a method and apparatus which optically isolates pixelregions to reduce optical crosstalk in a solid state image sensor.

BACKGROUND OF THE INVENTION

There are a number of different types of semiconductor-based imagers,including charge coupled devices (CCD's), photodiode arrays, chargeinjection devices (CID's), hybrid focal plane arrays, and complementarymetal oxide semiconductor (CMOS) imagers. Current applications ofsolid-state imagers include cameras, scanners, machine vision systems,vehicle navigation systems, video telephones, computer input devices,surveillance systems, auto focus systems, star trackers, motion detectorsystems, image stabilization systems, and other image acquisition andprocessing systems.

CMOS imagers are well known. CMOS images are discussed, for example, inNixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEEJournal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996);Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions onElectron Devices, Vol. 41(3), pp. 452-453 (1994); and are also disclosedin U.S. Pat. Nos. 6,140,630, 6,204,524, 6,310,366, 6,326,652, 6,333,205,and 6,326,868; assigned to Micron Technology, Inc., the entiredisclosures of which are incorporated herein by reference.

Semiconductor imaging devices include an array of pixel cells, whichconverts light energy received, through an optical lens, into electricalsignals. Each pixel cell contains a photosensor for converting arespective portion of a received image into an electrical signal. Theelectrical signals produced by the array of photosensors are processedto render a digital image.

The amount of charge generated by the photosensor corresponds to theintensity of light impinging on the photosensor. Accordingly, it isimportant that all of the light directed to the photosensor impinges onthe photosensor rather than being reflected or refracted toward anotherphotosensor as optical crosstalk.

For example, optical crosstalk may exist between neighboringphotosensors in a pixel array. In an ideal imager, ideally, all theincident photons on top of a microlens are directed towards thephotosensing element underneath that microlens. In reality, some of thephotons get refracted and reach adjacent photosensors. This leads toundesirable optical crosstalk between neighboring pixels. This problemgets worse with scaled pixels and as the distance between thephotosensor and the microlens increases. Increasing the number ofinterconnect metal layers typically increases this distance.

Optical crosstalk can bring about undesirable results in the imagesproduced by the imaging device. The undesirable results can become morepronounced as the density of a pixel cell in imager arrays increases,and as pixel cell size correspondingly decreases. The shrinking pixelcell sizes make it increasingly difficult to properly focus incominglight on the photosensor of each pixel cell without accompanying opticalcrosstalk.

Optical crosstalk can cause a blurring or reduction in contrast inimages produced by the imaging device. Optical crosstalk also degradesthe spatial resolution, reduces overall sensitivity, causes colormixing, and leads to image noise after color correction. As noted above,image degradation can become more pronounced as pixel cell and devicesizes are reduced. Furthermore, degradation caused by optical crosstalkis more conspicuous at longer wavelengths of light. Light having longerwavelengths penetrates more deeply into the silicon structure of a pixelcell, providing more opportunities for the light to be reflected orrefracted away from its intended photosensor target.

One proposal to reduce optical crosstalk provides a continuous air-gaparound the optical path to a photosensor. See Dun-Nian Yaung et al.,Air-Gap Guard Ring for Pixel Sensitivity and Crosstalk Improvement inDeep Sub-micron CMOS Image Sensor, PROC. OF IEDM, 2003; see also T. H.Hsu et al., Light Guide for Pixel Cross Talk Improvement in DeepSubmicron CMOS Image Sensor, IEEE ELECTRON DEVICE LETTERS, vol. 25, no.1, 2004, at 22-24. FIG. 1 represents a cross sectional view of an imagershowing two exemplary prior art techniques for dealing with opticalcrosstalk. The FIG. 1 imager has an air-gap guard ring 221 surrounding aphotosensor optical path 223 existing between a micro-lens 240 and aphotosensor 220. The air gap ring 221 is shown as being fabricated inthe lower metallization layers M2 of an imager. The air gap provides arefraction index difference between the air gap (n₂=1) and thesurrounding dielectric layers (n₁=1.4−1.6) and thus, the majority ofincident light will be collected in the targeted pixel cell due to thetotal internal reflection in the air-gap/dielectric film interface.However, the presence of an air gap ring 221 is not ideally suited forsolid state imagers. There are several reliability issues with the airgap such as its structural instability. Also, the color filter array(CFA) process, widely used in color imager fabrication, is known to havemetallic and mobile ion contaminants that might easily diffuse throughthe air gaps and affect the devices and photosensor characteristics inthe underlying pixel circuit.

Alternatively as also shown in FIG. 1, planar metal-shielding 225provided in an upper metallization layer M4 has been used in an effortto reduce optical crosstalk, but these may degrade pixel sensitivityand/or are not suitable for use in zooming lens systems.

Another method of reducing optical crosstalk uses optical waveguides.Optical waveguides are structures used for spatially confining anddirecting light onto the intended target. For instance, opticalwaveguides can be used to reduce the detrimental affects associated withlight shields such as light piping and light shadowing. Opticalwaveguides, however, are not widely used to focus light directly ontothe photosensor in imaging devices. Moreover, currently employed opticalwaveguide structures, require additional processing steps, adding to thecomplexity and costs of imager fabrication.

Accordingly, there is a need and desire for an improved apparatus andmethod for reducing optical crosstalk in imaging devices. There is alsoa need to more effectively and accurately increase overall pixelsensitivity and provide improved optical crosstalk immunity withoutadding complexity to the manufacturing process and/or increasingfabrication costs.

BRIEF SUMMARY OF THE INVENTION

Exemplary embodiments of the invention provide an optical guidestructure for a pixel which guides incoming light onto the photosensorof the pixel. The optical guide structure has an optically reflectingbarrier that mitigates against optical crosstalk. The optical guidestructure is made of low dielectric constant material with an index ofrefraction that is less than the index of refraction of the material ofsurrounding layers. This difference in refractive index causes aninternal reflection into an optical path existing between a lens andpixel.

In other exemplary embodiments, materials with high reflectivity such asmetals can be used to implement the optical guide structure. In yetanother embodiment, to improve the difference in the index of refractionbetween the fill material and the surrounding material, the surroundinglayers may be formed with materials having a relatively high index ofrefraction

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the invention will become moreapparent from the detailed description of exemplary embodiments providedbelow with reference to the accompanying drawings in which:

FIG. 1 illustrates a cross sectional view of a prior art pixel;

FIG. 2 is a plan view of an image sensor pixel constructed in accordancewith an exemplary embodiment of the invention;

FIG. 3 shows a cross sectional view of the image sensor pixel of FIG. 2,constructed in accordance with the exemplary embodiment of theinvention;

FIG. 4 shows the optical guide structure in accordance with theexemplary embodiment of the invention;

FIG. 5 shows the optical guide structure in accordance with anotherexemplary embodiment of the invention;

FIG. 6 shows the optical guide structure in accordance with anotherexemplary embodiment of the invention;

FIG. 7 shows the optical guide structure in accordance with theexemplary embodiment of the invention within a CMOS process;

FIG. 8 shows the optical guide structure in accordance with anotherexemplary embodiment of the invention within a CMOS process;

FIG. 9 shows the optical guide structure in accordance with anotherexemplary embodiment of the invention within a CMOS process;

FIG. 10 shows a CMOS image sensor constructed in accordance with anembodiment of the invention.

FIG. 11 shows a processor system incorporating at least one imagerconstructed in accordance with the exemplary embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, reference is made to theaccompanying drawings which form a part hereof, and in which is shown byway of illustration specific embodiments by which the invention may bepracticed. It should be understood that like reference numeralsrepresent like elements throughout the drawings. These exemplaryembodiments are described in sufficient detail to enable those skilledin the art to practice the invention. It is to be understood that otherembodiments may be utilized, and that structural, logical and electricalchanges may be made without departing from the spirit and scope of thepresent invention.

The terms “wafer” and “substrate” are to be understood as including allforms of semiconductor wafers and substrates including silicon,silicon-on-insulator (SOI), silicon-on-sapphire (SOS), doped and undopedsemiconductors, epitaxial layers of silicon supported by a basesemiconductor foundation, and other semiconductor structures.Furthermore, when reference is made to a “wafer” or “substrate” in thefollowing description, previous process steps may have been utilized toform regions or junctions in the base semiconductor structure orfoundation. In addition, the semiconductor need not be silicon-based,but could be based on other semiconductors, for example,silicon-germanium, germanium, or gallium arsenide.

The term “pixel” refers to a picture element unit cell containingcircuitry including a photosensor and semiconductors for convertingelectromagnetic radiation to an electrical signal. For purposes ofillustration, fabrication of one or more representative pixels is shownand described. Typically, fabrication of all pixels in an imager willproceed simultaneously in a similar fashion.

Although the invention is described herein with reference to thearchitecture and fabrication of one or a limited number of pixels, itshould be understood that this is representative of a plurality of pixelcells as typically would be arranged in an imager array having pixelcells arranged, for example, in rows and columns.

In addition, although the invention is described below with reference toa pixel for a CMOS imager, the invention has applicability to othersolid-state imaging devices using pixels (e.g., a CCD or other solidstate imager).

The invention may also be employed in display devices where a pixel hasa light emitter for emitting light. The following detailed descriptionis, therefore, not to be taken in a limiting sense, and the scope of thepresent invention is defined only by the appended claims.

Referring to the FIG. 2 top down view and the FIG. 3 cross sectionalview, embodiments of the invention provide a trench 350 filled with amaterial 351 designed to provide internal reflection within the opticalpath 223. The trench 350 and filling material 351 are formed in opticalpath 223 between a photosensor layer 220 formed on a substrate and amicro lens layer 240 for each pixel cell. The trench 350 and associatedfill material 351 surround the optical path between a lens and acorresponding photosensor, which corresponds to a lateral area definedby the photosensor. The depth and width of the filled trench 350 can betailored depending on the need, and may extend from the upper layers ofthe image sensor, for example, beginning at the color filter array 250level, to below a bottom ILD layer 210 into the protective layer 230,formed typically of BPSG, provided above the active area of thephotosensor layer 220. The trench can be etched by any method known inthe art.

In a first embodiment of the invention, the trench 350 is filled with alow-dielectric constant material 440 (low-k material), having adielectric constant below 1.45. The low dielectric constant material 440within trench 350 has an index of refraction that is less than the indexof refraction of the material used for the surrounding imager layersshown as the BPSG layer 230, ILD layers with associated metallization210, passivation TEOS layer 260, and CFA layer 250.

For example, the dielectric of the ILD layer 210 is typicallyimplemented by depositing amorphous silicon dioxide, whose index ofrefraction is approximately between 1.45 and 1.54. Thus, in thisexample, a material with a lower index of refraction than 1.45 will fillthe trench 350. Additionally, there are numerous other low-k polymers(discussed below) that can be used for the fill material 351, as long astheir respective reflective index is below that of the surroundinglayers.

FIG. 4 illustrates the FIG. 3 structure prior to formation of an upperpassivation layer 270 and micro-lens layer 240. The trench 350 may beetched through the CFA layer 250, passivation layer 260, ILD andassociated metallization layers 210 and partially into the BPSG layer230. The trench is then filled with the fill material 351 and CMPplanarized, after which the passivation layer 270 and micro-lens layers240 are added.

FIG. 5 illustrates a modification of the embodiment shown in FIG. 4 inwhich the trench contains multiple layers of fill material 351′. Twofill material 351′ layers 670, 780 are shown in FIG. 6. FIG. 6illustrates a fill material 351″ formed of these layers 440, 670, 780 ofdifferent materials. However, the number of layers used for the fillmaterial in trench 350 is in no way limited by these examples.

It should be appreciated that in the exemplary embodiment discussedabove the trench 350 has been described as extending to and through theILD layer 210, passivation layer 260, and CFA layer 250, however it maybe extended from or continue into additional layers. For examplereferring back to FIG. 3, trench 350 may begin at the level ofmicro-lens layer 240, or at the level of passivation layer 270. In otherwords, the trench 350 may extend through any other layer included withinthe image sensor between the photosensor layer 220 and the micro-lenslayer 240. The invention may be used in solid state imagers employingvarious kinds of photosensors formed on a substrate in photosensor layer220, including but not limited to photodiodes, photo transistors,photoconductors, and photogates.

In all of the described embodiments, there is a difference in refractiveindex between the surrounding film material (refractive index=n₁) andthe material 351 (FIG. 4), 351′ (FIG. 5), 351″ (FIG. 6) used to fill thetrench 350 (refractive index=n₂). If n₁ is greater than n₂, for largeangles of incidence of the incident light there is total internalreflection, and a considerable reduction in optical crosstalk.

In general, low dielectric constant materials will provide lowrefractive indexes. The various exemplary embodiments may use variousmaterials alone (FIG. 4), or in combination (FIGS. 5, 6) as the fillmaterial 351, 351′, 351″ within the trench 350 such as those that havepredominantly oxide properties such as SiO₂/TEOS, Spin-cn-dielectricoxide (SOD), carbon doped silicon di-oxides, fluorinated silica glassoxide (FSG), etc. However, other commercially available materials canalso be used such as SiLK, FLARE 2.0, Black Diamond Corel, PSiLK, Orion,LKD 5109 and XPX. It should be appreciated that this list of materialsis in no way exhaustive of possible materials that can be used forfilling the trench 350 of the optical guide structure, as the importantpoint is that the index of refraction of the trench 350 fill material islower than that of the material layers surrounding trench 350 along theoptical path 223. It should be understood that any optical transparentmaterial with a suitable index of refraction can be used as ILD materialand trench fill material as long as the fill material used in the trenchhas a lower index of refraction compared to the material surrounding thetrench. It should also be understood that any optical transparentmaterial with a suitable index of refraction can be used as ILD materialand trench fill material as long as the fill material used in the trenchhas a higher index of refraction compared to the material surroundingthe trench. The desired reflection properties of the optical guide cancome about from any such difference, higher or lower, in the index ofrefraction from that of the surrounding layers and that of the opticalguide.

Additionally, the typical materials used for the various layers 210,260, 250, 270 may have a relatively low index of refraction. To improvethe difference in the index of refraction between the fill material 351,351′, 351″ and the surrounding material, the surrounding layers, e.g.,210, 260, 250, 270 may be formed with materials having a relatively highindex of refraction, thus expanding the number of possible materialshaving a lower index of refraction, which can be used for the trench 350fill material.

In another embodiment of the invention, fill materials with high lightreflectivity such as metals may also be used to fill the trench 350.Some metals have a very high light reflectivity such as aluminum,copper, silver and gold and can effectively serve as an optical barriermaterial. It should be appreciated that the metals mentioned are in noway an exhaustive list of possible metals which can be used, and metalalloys may also be used. The metal fill material may be used alone asfill material 351 (FIG. 4), or it may be layered fill material 351′,351″ in the trench 350, as shown in FIGS. 5, 6. In addition, one or morereflective metal layers may be used in a layer of combination withintrench 350 with layers of the non-metal materials discussed above.

FIG. 7 illustrates an embodiment in which the trench 350 fill materialis not planarized after the trench fill process. Using the FIG. 4multi-layer fill 351′ as an example, after trench 350 is etched into thevarious layers and filled with the low-dielectric constant materials,the fill materials are not planarized leaving the fill materialsextending from the outer edges of the trench 350 over the layer 250. Apatterned metal layer 940 may also be placed above the trenches 350,alone or multiple levels, as shown in FIG. 8 to serve as an etch stoplayer for subsequent processing and/or to also provide a light shield.FIG. 9 shows a top down view of FIG. 8 with the patterned metal layer940.

FIG. 10 illustrates an exemplary CMOS imager 1100 that may utilize theinvention. The CMOS imager 1100 has a pixel array 1105 comprising pixelsconstructed to include the overlying optical structure in accordancewith the invention. The CMOS pixel array circuitry and potation areconventional and are only briefly described herein. Each pixel of thearray includes a known four transistor (4T) pixel. Array row lines areselectively activated by a row driver 1110 in response to row addressdecoder 1120. A column driver 1160 and column address decoder 1170 arealso included in the imager 1100. The imager 1100 is operated by thetiming and control circuit 1150, which controls the address decoders1120, 1170.

A sample and hold circuit 1161 associated with the column driver 1160reads a pixel reset signal Vrst and a pixel image signal Vsig forselected pixels. A differential signal (Vrst−Vsig) is amplified bydifferential amplifier 1162 for each pixel and is digitized byanalog-to-digital converter 1175 (ADC). The analog-to-digital converter1175 supplies the digitized pixel signals to an image processor 1180which forms a digital image.

FIG. 11 shows a system 1200, a typical processor system modified toinclude an imaging device 1210 (such as the imaging device 1100illustrated in FIG. 10) of the invention. The processor system 1200 isexemplary of a system having digital circuits that could include imagesensor devices. Without being limiting, such a system could include acomputer system, camera system, scanner, machine vision, vehiclenavigation, video phone, surveillance system, auto focus system, startracker system, motion detection system, image stabilization system, andother systems employing an imager.

System 1200, for example a camera system, generally comprises a centralprocessing unit (CPU) 1220, such as a microprocessor, that communicateswith an input/output (I/O) device 1270 over a bus 1280. Imaging device1210 also communicates with the CPU 1220 over the bus 1280. Theprocessor-based system 1200 also includes random access memory (RAM)1290, and can include removable memory 1230, such as flash memory, whichalso communicate with the CPU 1220 over the bus 1280. The imaging device1210 may be combined with a processor, such as a CPU, digital signalprocessor, or microprocessor, with or without memory storage on a singleintegrated circuit or on a different chip than the processor.

It should be appreciated that there are likely many alternatives formaterials that may be suitably employed to provide the optical guide forintegrated image sensors including metals, polymers, semiconductors, anddielectric. This is especially true if a material other than amorphoussilicon dioxide is used in the ILD layers.

The processes and devices described above illustrate preferred methodsand typical devices of many that could be used and produced. The abovedescription and drawings illustrate embodiments, which achieve theobjects, features, and advantages of the present invention. However, itis not intended that the present invention be strictly limited to theabove-described and illustrated embodiments. Any modification, thoughpresently unforeseeable, of the present invention that comes within thespirit and scope of the following claims should be considered part ofthe present invention.

1-53. (canceled)
 54. An optical guide structure comprising: a trenchformed within a first material to continuously surround a lateral areaof a photo-conversion device, the trench enclosing an area of the firstmaterial and defining an optical path through the area, the trench beingfilled with a second material having a property of producing reflectionof light within the optical guide structure, the first material having alower refractive index than the second material.
 55. The optical guidestructure of claim 54, wherein the second material has a dielectricconstant less than 1.45.
 56. The optical guide structure of claim 54,wherein the second material comprises carbon doped silicon dioxides. 57.The optical guide structure of claim 54, wherein the second materialcomprises fluorinated silica glass oxide.
 58. The optical guidestructure of claim 54, wherein the second material comprises a metal.59. The optical guide structure of claim 58, wherein the metal comprisessilver or copper.
 60. The optical guide structure of claim 54, whereinthe trench forms a ring-shaped optical guide structure.
 61. The opticalguide structure of claim 54, wherein the trench extends above a surfaceof the first material into a third material.
 62. The optical guidestructure of claim 61 wherein the third material has a lower refractiveindex than the second material.
 63. The optical guide structure of claim62, wherein first material and third material have the same refractiveindex.
 64. The optical guide structure of claim 61, wherein the thirdmaterial comprises a color filter array.
 65. The optical guide structureof claim 61, wherein the third material comprises a boron-phosphoroussilicon glass material.
 66. The optical guide structure of claim 54,wherein the second material comprises an interlayer dielectric material.67. The optical guide structure of claim 55, wherein the first materialhas a dielectric constant which is greater than 1.45.
 68. The opticalguide structure of claim 54, wherein the first material has a refractiveindex in between 1.45 and 1.54 and the second material has a refractiveindex less than 1.45.
 69. An optical guide structure comprising: atrench, formed above a photo-conversion device and within at least onematerial to define the optical guide structure, the trench forming acontinuous structure surrounding a lateral area above thephoto-conversion device, where an optical path is defined by an areasurrounded by the trench, the trench being filled with a plurality ofother materials having a higher refractive index than the at least onematerial, the filled trench producing reflection of light within theoptical guide structure.
 70. The optical guide structure of claim 69,wherein the trench symmetrically surrounds the lateral area of the photoconversion device.
 71. The optical guide structure of claim 69, whereinthe optical path is centered over the photo conversion device and has across sectional area defined by the photo conversion device.
 72. Theoptical guide structure of claim 69, wherein each of the plurality ofother materials has a dielectric constant below 1.45.
 73. The opticalguide structure of claim 69, wherein the at least one material has arefractive index in between 1.45 and 1.54 and the plurality of othermaterials have a refractive index less than 1.45.